This invention relates to a magnetron drive power supply with a magnetron of a microwave oven, etc., as a load.
A magnetron drive power supply in a related art will be discussed with reference to the accompanying drawings (FIGS. 8 and 9). FIG. 8 is a circuit block diagram of a magnetron drive power supply in a related art. A semiconductor switch in a high-frequency inverter 2 is controlled by controller 7, whereby a commercial power supply 1 is converted into radio-frequency power of 20 to 50 kHz and the radio-frequency power is supplied to a high-voltage transformer 3. A high-voltage rectification circuit 4 and a magnetron 5 are connected to the secondary side of the high-voltage transformer 3 and a DC high voltage is applied to the magnetron 5 for generating a 2.45-GHz radio wave.
Zero-voltage detector 6 detects a zero voltage point of the power supply voltage 1 and causes modulation signal generator 9 to generate a modulation wave form responsive to the power supply phase. Upon reception of input of zero voltage detection from the zero-voltage detector 6, the modulation signal generator 9 outputs a modulation waveform of one period of the power supply voltage 1 as a peak value responsive to the setup value of input current. Using such a modulation signal, the controller 7 can control the input current to the form close to a sine wave. The controller 7 performs 20 to 50-kHz PWM modulation of the modulation signal by oscillator 10 and transmits the signal to driver 8, thereby controlling the on-duration of the semiconductor switch in the high-frequency inverter 2. As the zero-voltage detector 6, voltage detection with a transformer using a photocoupler, etc. is available. And, as the controller 7, control of a microcomputer, etc., is used.
FIGS. 9A to 9D are waveform charts of the magnetron drive power supply in the related art. Upon reception of a signal of commercial power supply (FIG. 9A), a signal of zero voltage detection (FIG. 9B) oscillated at the timing of zero voltage is output by the zero-voltage detector 6. The rising edge of the signal of the zero-voltage detector 6 is detected and a modulation signal (FIG. 9C) preset so that the input current becomes a predetermined value and moreover the power factor of the input current becomes close to 1 is output for one period of the commercial power supply 1. The modulation signal (FIG. 9C) is compared with the oscillation frequency of oscillator output (FIG. 9D) by comparator 11, where by the signal is subjected to PWM modulation and is supplied to the driver 8 as a drive signal. The modulation signal is set so that the frequency of the semiconductor switch in the high-frequency inverter 2 becomes 20 to 50 kHz. The controller 7 performs such control, whereby electric power having a current waveform with a less harmonic component with a good power factor can be supplied.
However, in the magnetron drive power supply in the related art, if the zero voltage detection shifts due to noise, instantaneous power interruption, etc., the modulation waveform deviates from the essential timing and a possibility of leading to a failure of the high-frequency inverter because of overvoltage, overcurrent, etc., occurs.